Sources systems are all the data feeding pipes to the Staging Area. TYPICALLY any Transformation on the data is done after the data in its raw OR unchanged form is picked from the source systems. The further details of the Source Systems can be Data Warehouse Design & Architecture in Data Warehousing
Core source Systems
These system include the core systems mostly having well organized database, set schedules, the data being updated on online basis. Typical systems are core product manufacturing systems, accounting systems, Money Management systems, commission/Sales Compensation Systems, tightly coupled job/workflow systems.
Field OR Front-end Source Systems
These systems, include the systems, which are primarily used by theCustomer acquisition and retention staff. These include Customer service systems, Sales automation systems, leads management systems, campaign management systems.
Modeling and Analysis systems
This family of systems include budgeting, planning, forecasting, pricing and valuation type of system.
External Data
This is the data, which you receive from regulators, credit bureau, medical bureau, industry associations, market research firms, database marketing companies and other sources. This data (unlike data provided by your suppliers and Customers;which goes into your core OR field systems) follows a standard format generally governed by the supplier.
Non-Data Base/Desk top Sources
Wealth of information and critical operational data resides in the excels and MS access tables in the desktops OR local servers of organization. If you want to shock yourself, just make a study of the number of excel based applications, which have become critical part of operational delivery and reporting. The numbers could go in hundreds, if not in thousands.
Off-line Databases
As no organization data management strategy goes through a systematic & planned growth, it is possible that you might have some offline database used by the users to generate their reports. For the sake of speed, one may tend to use that as a source system. However, in a medium to long run, this may become counter productive because you will make that offline database redundant.